1. Field of the Invention
The present invention relates to an apparatus and a method for non-invasively measuring bio-fluid concentrations. More particularly, the present invention relates to an apparatus and a method for non-invasively measuring bio-fluid concentrations using photoacoustic spectroscopy.
2. Description of the Related Art
Even though research has long been carried out worldwide on a method of measuring glucose levels by means of light without actually collecting blood, such research has failed to provide any distinctive results.
Various measurement techniques, such as near infrared absorption, far infrared absorption, Ramann spectroscopy, polarization rotation, Stimulate Ramann, dispersion measurement, temperature measurement, statistical analysis, and pretreatment research, have been adopted in vivo measure bio-fluid concentrations. However, since each of these conventional measurement techniques has several disadvantages, the in vivo measurement of bio-fluid concentrations has not been satisfactorily accomplished.
For example, near infrared absorption has the following disadvantages. First, an absorption peak may not exist at a predetermined frequency. Second, the absorption bands of components may overlap one another. Third, it is difficult to anticipate the concentration of a substance having a low concentration because dispersion easily occurs due to biological tissues. In the case of far infrared absorption, far infrared rays are barely able to penetrate the human body, even though they cause dispersion less frequently, and there exists a distinct absorption peak. In the case of Ramann spectroscopy or polarization rotation, dispersion occurs frequently due to the existence of many dispersion factors in the human body, and thus it is difficult to precisely measure bio-fluid concentrations.
Recently, intensive research has been carried out on an apparatus and method for bio-fluid concentrations measurement by means of photoacoustic spectroscopy. When light enters a test sample, molecules are excited and collide with one another, thereby generating heat. The change of heat causes the change of pressure in an airtight container, which generates an acoustic signal, i.e., a sound wave. The sound wave can then be detected using a microphone.
FIGS. 1 and 2 are diagrams showing a non-invasive photoacoustic measurement device according to the prior art. Referring to FIG. 1, a conventional non-invasive photoacoustic measurement device 10 includes an excitation source 12, a controller/modulator 14, a probe 16, a lock-in amplifier 18, and a processor 20.
In operation, the excitation source 12 generates a sound wave when the excitation source 12 is irradiated onto a biological tissue, such as skin. The sound wave is transmitted to the human body through a transmitter 22, such as a bundle of optic fibers.
The probe 16, as shown in greater detail in FIG. 2, includes a measurement cell 26, a reference cell 28, a window 30, and a differential microphone 32. The sound wave, generated when the excitation source 12 is irradiated onto a tissue 24, passes through the window 30 of the measurement cell 26 and heats air 38 in contact with the tissue 24 in the measurement cell 26 on a regular basis with the same modulated frequency as that of the sound wave. The sound wave is absorbed into a targeted component of the tissue 24, and the air in the measurement cell 26 repeatedly contracts and expands due to the periodic variation of the temperature. As a result, a periodic sound wave having the same modulated frequency as that of the sound wave is generated.
The periodic sound wave inside the measurement cell 26 is detected by the differential microphone 32, a first end 40 of the differential microphone 32 is located in the measurement cell 26 and a second end 42 of the differential microphone 32 is located in the reference cell 28. The measurement cell 26 is located on a first predetermined surface 46 of the tissue 24, onto which laser beams are irradiated. The reference cell 28 is located on a second predetermined surface 48 of the tissue 35, onto which no laser beams are irradiated.
The signals detected by the probe 16 become the outputs of the differential microphone 32 and are transmitted to the lock-in amplifier 18. Among the outputs, the lock-in amplifier 18 extracts only signals of the same frequency as the modulated frequency of the light beams that are generated and irradiated from the excitation source 12 under the control of the controller/modulator 14. The processor 20 analyzes the frequencies of the signals extracted by the lock-in amplifier 18 and derives a polarized acoustic spectrum. The conventional acoustic measurement device determines the concentration of a targeted component based on this polarized acoustic spectrum.
Even though the reference cell 28 attempts to compensate for noise generated by the human body, such as muscular movements, the conventional photoacoustic measurement device illustrated in FIGS. 1 and 2 is not able to precisely represent the state of the human body because the device senses only modulated signals and the signals themselves have predetermined frequency bands.
The aforementioned conventional bio-fluid measurement device using photoacoustic spectroscopy detects infrared laser beams among all laser beams irradiated on a predetermined material from a semiconductor laser, using a photoacoustic detector. Next, the bio-fluid measurement device analyzes bio-fluid concentrations based on acoustic signals detected by the photoacoustic detector. However, due to the fact that the characteristics of transmission of sound waves may vary depending on the person being measured and the body part of the person being measured, this conventional bio-fluid measurement device is not able to measure precisely bio-fluid concentrations, which is similarly a problem with other conventional measurement devices using photoacoustic spectroscopy.